Reciprocal channel sounding reference signal multiplexing

ABSTRACT

Systems and techniques are disclosed to enhance the efficiency of available bandwidth between UEs and base stations. A UE transmits a sounding reference signal (SRS) to the base station. The base station characterizes the uplink channel based on the SRS received and, using reciprocity, applies the channel characterization for the downlink channel. As part of applying the channel information, the base station forms the beam to the UE based on the uplink channel information obtained from the SRS. The UE may include an array of antennas, each UE transmitting a different SRS that the base station receives and uses to characterize the downlink. Multiple UEs (or a single UE with multiple antennas) transmit SRS at the same time and frequency allocation (non-orthogonal), but with each sending its own unique SRS. Further, multiple UEs (or a single UE with multiple antennas) may send their SRS at unique time/frequency allocations (orthogonal).

CROSS REFERENCE TO RELATED APPLICATIONS & PRIORITY CLAIM

The present application is a continuation application of U.S.Non-Provisional patent application Ser. No. 14/866,778, filed Sep. 25,2015, which claims priority to and the benefit of the U.S. ProvisionalPatent Application No. 62/133,334, filed Mar. 14, 2015, the disclosureof each of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This application relates to wireless communication systems, and moreparticularly to using channel state information obtained from an uplinksounding signal in non-orthogonal or orthogonal applications to beamformdownlink messages to targeted recipients.

INTRODUCTION

A wireless communication network may include a number of base stationsthat can support communication for a number of user equipments (UEs). Inrecent years, the carrier frequencies at which base stations and UEscommunicate have continued to increase and include larger bandwidths. Totake advantage of these higher frequencies, more antennas in the samephysical space have been used. For these higher frequency bands to beuseful and approximate the same coverage radius as prior technologies(such as 2G, 3G, or 4G), however, more beam forming gain (and moreaccurate) is becoming necessary.

Further, conventional systems employ various types of reference signals,with varying fixed structures, to provide sufficient measurements andestimations for adaptive multi-antenna operation in uplink and/ordownlink directions. For example, a channel state information referencesignal (CSI-RS) may be used on a downlink from the base station to aidthe base station in beam form determination, an uplink demodulationreference signal (DM-RS) specific to each UE may be used to estimatechannel information for the uplink specifically, and each UE may use asounding reference signal (SRS) on the uplink to aid in scheduling(e.g., determining which frequency bands are good or bad for data).There is no single signal that is able to achieve all of abovefunctionality for UEs.

Reciprocity describes the ability for a station to use information (suchas a multipath delay profile) from one channel (e.g., the uplink) inmaking determinations regarding another channel (e.g., the downlink).Reciprocity has not been available for cellular networks because currentapproaches require reference signals specific for particular antennas,such as CSI-RS in the long term evolution (LTE) context. Further, CSI-RSand other types of signals do not scale well, which is becoming anever-increasing issue as the demand for mobile broadband continues toincrease.

BRIEF SUMMARY OF SOME EXAMPLES

The following summarizes some aspects of the present disclosure toprovide a basic understanding of the discussed technology. This summaryis not an extensive overview of all contemplated features of thedisclosure, and is intended neither to identify key or critical elementsof all aspects of the disclosure nor to delineate the scope of any orall aspects of the disclosure. Its sole purpose is to present someconcepts of one or more aspects of the disclosure in summary form as aprelude to the more detailed description that is presented later.

In one aspect of the disclosure, a method includes receiving, at a basestation, a sounding reference signal (SRS) from a user equipment (UE)via an uplink channel; obtaining, by the base station, information fromthe SRS about the uplink channel and applying the information to adownlink channel; and transmitting, from the base station, a beamformeddownlink communication to the UE via the downlink channel based on theinformation obtained from the SRS.

In an additional aspect of the disclosure, a method for communicatingwith a base station includes transmitting, from a plurality of userequipments (UEs), a plurality of sounding reference signals (SRSs),wherein the plurality of SRSs are transmitted using non-orthogonalphysical resources via corresponding uplink channels; and receiving,from the base station, beamformed downlink communications based oninformation obtained from the plurality of SRSs about the uplinkchannels and applied to downlink channels.

In an additional aspect of the disclosure, a method for communicatingwith a base station includes arranging, at a user equipment (UE)comprising a plurality of antennas, a different sounding referencesignal (SRS) corresponding to a different one of the plurality ofantennas; transmitting, from the UE, a different sounding referencesignal (SRS) from each one of the multiple antennas; and receiving, fromthe base station, beamformed downlink communications based oninformation obtained from the different SRS corresponding to each one ofthe multiple antennas.

In an additional aspect of the disclosure, a method for communicatingwith a plurality of user equipments (UEs) includes receiving, at a basestation, a plurality of sounding reference signals (SRSs), one from eachUE from among the plurality of UEs, wherein each SRS is transmitted fromeach respective UE using orthogonal physical resources; obtaining, bythe base station, information from each SRS about the respective uplinkchannel and applying the information to a respective downlink channel;and transmitting, from the base station, a beamformed downlinkcommunication to each UE via the respective downlink channel based onthe information obtained from each SRS.

In an additional aspect of the disclosure, a method for communicatingwith a base station includes transmitting, from a user equipment (UE),multiple narrowband sounding reference signals (SRSs) at differentfrequency sub-bands during one subframe; and receiving, from the basestation, a beamformed downlink communication based on informationobtained from the SRSs corresponding to each of the different frequencysub-bands.

In an additional aspect of the disclosure, a base station includes atransceiver configured to receive a sounding reference signal (SRS) froma user equipment (UE) via an uplink channel; and a processor configuredobtain information from the SRS about the uplink channel and apply theinformation to a downlink channel, wherein the transceiver is furtherconfigured to transmit a beamformed downlink communication to the UE viathe downlink channel based on the information obtained from the SRS.

In an additional aspect of the disclosure, a user equipment includes aplurality of antennas; a processor configured to arrange a differentsounding reference signal (SRS) corresponding to a different one of theplurality of antennas; and a transceiver configured to transmit adifferent sounding reference signal (SRS) from each one of the multipleantennas to a base station and receive, from the base station,beamformed downlink communications based on information obtained fromthe different SRS corresponding to each one of the multiple antennas.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a base station to receive a sounding reference signal(SRS) from a user equipment (UE) via an uplink channel; code for causingthe base station to obtain information from the SRS about the uplinkchannel and applying the information to a downlink channel; and code forcausing the base station to transmit a beamformed downlink communicationto the UE via the downlink channel based on the information obtainedfrom the SRS.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a user equipment (UE) comprising a plurality ofantennas to arrange a different sounding reference signal (SRS)corresponding to a different one of the plurality of antennas; code forcausing the UE to transmit a different sounding reference signal (SRS)from each one of the multiple antennas to a base station; and code forcausing the UE to receive, from the base station, beamformed downlinkcommunications based on information obtained from the different SRScorresponding to each one of the multiple antennas.

In an additional aspect of the disclosure, a base station includes meansfor receiving a sounding reference signal (SRS) from a user equipment(UE) via an uplink channel; means for obtaining information from the SRSabout the uplink channel and applying the information to a downlinkchannel; and means for transmitting a beamformed downlink communicationto the UE via the downlink channel based on the information obtainedfrom the SRS.

In an additional aspect of the disclosure, a user equipment (UE)comprising a plurality of antennas includes means for arranging adifferent sounding reference signal (SRS) corresponding to a differentone of the plurality of antennas; means for transmitting a differentsounding reference signal (SRS) from each one of the multiple antennasto a base station; and means for receiving, from the base station,beamformed downlink communications based on information obtained fromthe different SRS corresponding to each one of the multiple antennas.

Other aspects, features, and embodiments of the present invention willbecome apparent to those of ordinary skill in the art, upon reviewingthe following description of specific, exemplary embodiments of thepresent invention in conjunction with the accompanying figures. Whilefeatures of the present invention may be discussed relative to certainembodiments and figures below, all embodiments of the present inventioncan include one or more of the advantageous features discussed herein.In other words, while one or more embodiments may be discussed as havingcertain advantageous features, one or more of such features may also beused in accordance with the various embodiments of the inventiondiscussed herein. In similar fashion, while exemplary embodiments may bediscussed below as device, system, or method embodiments it should beunderstood that such exemplary embodiments can be implemented in variousdevices, systems, and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication network in accordance withvarious aspects of the present disclosure.

FIG. 2 illustrates a wireless communication network which uses soundingreference signals to enable beamforming at a base station.

FIG. 3 illustrates an exemplary subframe structure.

FIG. 4 illustrates an exemplary frame structure for a synchronoussubframe system with periodic channel decorrelation.

FIG. 5 illustrates an exemplary frame structure for a synchronoussubframe system with random channel decorrelation.

FIG. 6 illustrates an exemplary subframe structure for multiplexed SRSfrom a multi-antenna user equipment.

FIG. 7 illustrates an exemplary frame structure for an extended lengthSRS in a low-interference environment.

FIG. 8 illustrates an exemplary frame structure for an extended lengthSRS in a high-interference environment.

FIG. 9 is a flowchart illustrating an exemplary method for using anuplink sounding reference signal for channel estimation in accordancewith various aspects of the present disclosure.

FIG. 10 is a flowchart illustrating an exemplary method for using anuplink sounding reference signal for channel estimation in accordancewith various aspects of the present disclosure.

FIG. 11 is a flowchart illustrating an exemplary method for using anuplink sounding reference signal for channel estimation in accordancewith various aspects of the present disclosure.

FIG. 12 is a flowchart illustrating an exemplary method for using anuplink sounding reference signal for channel estimation in accordancewith various aspects of the present disclosure.

FIG. 13 is a block diagram of an exemplary wireless communicationdevice, such as a user equipment, according to embodiments of thepresent disclosure.

FIG. 14 is a block diagram of an exemplary wireless communicationdevice, such as a base station, according to embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA network may implement aradio technology such as Global System for Mobile Communications (GSM).An OFDMA network may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDMA, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). CDMA2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the wireless networks andradio technologies mentioned above as well as other wireless networksand radio technologies, such as a next generation (e.g., 5^(th)Generation (5G)) network.

Embodiments of the present disclosure introduce systems and techniquesto enhance the efficiency of use of available bandwidth in wirelesscommunications channels between UEs and base stations. In an embodiment,multiplexing may be used to aid in increasing the efficiency of use ofchannel resources, such as frequency division multiple access (FDMA),time division multiple access (TDMA), code division multiple access(CDMA), or spatial division multiple access (SDMA). One way of achievingSDMA, or space division multiplexing, is by use of beamforming. If adevice has multiple antennas, it may transmit signals from all antennasat once while altering the phase of the signal from each antenna toproduce constructive and destructive interference. The interference maybe calibrated to produce constructive interference in a specificdirection and destructive interference in all other directions, thusessentially transmitting a “beam” of information that does not createinterference in any other spatial area. Multiple beams may therefore betransmitted at once in different directions without interference. Inorder to successfully beamform, the multiple antenna device usesinformation about the channel between itself and its intended recipientdevice to create a beam which will reach the recipient.

Thus, according to embodiments of the present disclosure, a base stationmay harness channel reciprocity in order to use channel informationobtained from the uplink channel from a UE to the base station for thedownlink. A UE may transmit a sounding reference signal (SRS) to thebase station within a single subframe. The base station, in turn, maycharacterize the uplink channel based on the SRS received and, usingreciprocity, apply the same channel characterization for the downlinkchannel back to the UE. As part of applying the channel information tothe downlink, the base station may form the beam to the UE based on theuplink channel information obtained from the SRS.

In further embodiments, the UE may include an array of antennas (MIMO).In that situation, each UE may transmit a different SRS that the basestation receives and then uses for the downlink to those variousantennas (or, alternatively, multiple UEs with single antennas could beused to same effect). For example, multiple UEs (or a single UE withmultiple antennas) may transmit SRS at the same time and at the samefrequency allocation (e.g., non-orthogonal), but with each UE sendingits own unique SRS (based on unique scrambling codes or interleavingpermutations, for example). In another example, multiple UEs (or asingle UE with multiple antennas) may send their SRS at uniquetime/frequency allocations (orthogonal).

FIG. 1 illustrates a wireless communication network 100 in accordancewith various aspects of the present disclosure. The wirelesscommunication network 100 may include a number of UEs 102, as well as anumber of base stations 104. The base stations 104 may include anevolved Node B (eNodeB). A base station may also be referred to as abase transceiver station, a node B, or an access point. A base station104 may be a station that communicates with the UEs 102 and may also bereferred to as a base station, a node B, an access point, and the like.

The base stations 104 communicate with the UEs 102 as indicated bycommunication signals 106. A UE 102 may communicate with the basestation 104 via an uplink and a downlink. The downlink (or forward link)refers to the communication link from the base station 104 to the UE102. The uplink (or reverse link) refers to the communication link fromthe UE 102 to the base station 104. The base stations 104 may alsocommunicate with one another, directly or indirectly, over wired and/orwireless connections, as indicated by communication signals 108.

UEs 102 may be dispersed throughout the wireless network 100, as shown,and each UE 102 may be stationary or mobile. The UE 102 may also bereferred to as a terminal, a mobile station, a subscriber unit, etc. TheUE 102 may be a cellular phone, a smartphone, a personal digitalassistant, a wireless modem, a laptop computer, a tablet computer, etc.The wireless communication network 100 is one example of a network towhich various aspects of the disclosure apply.

Each base station 104 may provide communication coverage for aparticular geographic area. In 3GPP, the term “cell” can refer to thisparticular geographic coverage area of a base station and/or a basestation subsystem serving the coverage area, depending on the context inwhich the term is used. In this regard, a base station 104 may providecommunication coverage for a macro cell, a pico cell, a femto cell,and/or other types of cell. A macro cell generally covers a relativelylarge geographic area (e.g., several kilometers in radius) and may allowunrestricted access by UEs with service subscriptions with the networkprovider. A pico cell may generally cover a relatively smallergeographic area and may allow unrestricted access by UEs with servicesubscriptions with the network provider. A femto cell may also generallycover a relatively small geographic area (e.g., a home) and, in additionto unrestricted access, may also provide restricted access by UEs havingan association with the femto cell (e.g., UEs in a closed subscribergroup (CSG), UEs for users in the home, and the like). A base stationfor a macro cell may be referred to as a macro base station. A basestation for a pico cell may be referred to as a pico base station. Abase station for a femto cell may be referred to as a femto base stationor a home base station.

In the example shown in FIG. 1, the base stations 104 a, 104 b and 104 care examples of macro base stations for the coverage areas 110 a, 110 band 110 c, respectively. The base stations 104 d and 104 e are examplesof pico and/or femto base stations for the coverage areas 110 d and 110e, respectively. As will be recognized, a base station 104 may supportone or multiple (e.g., two, three, four, and the like) cells.

The wireless network 100 may also include relay stations. A relaystation is a station that receives a transmission of data and/or otherinformation from an upstream station (e.g., a base station, a UE, or thelike) and sends a transmission of the data and/or other information to adownstream station (e.g., another UE, another base station, or thelike). A relay station may also be a UE that relays transmissions forother UEs. A relay station may also be referred to as a relay basestation, a relay UE, a relay, and the like.

The wireless network 100 may support synchronous or asynchronousoperation. For synchronous operation, the base stations 104 may havesimilar frame timing, and transmissions from different base stations 104may be approximately aligned in time. For asynchronous operation, thebase stations 104 may have different frame timing, and transmissionsfrom different base stations 104 may not be aligned in time.

In some implementations, the wireless network 100 utilizes orthogonalfrequency division multiplexing (OFDM) on the downlink andsingle-carrier frequency division multiplexing (SC-FDM) on the uplink.OFDM and SC-FDM partition the system bandwidth into multiple (K)orthogonal subcarriers, which are also commonly referred to as tones,bins, or the like. Each subcarrier may be modulated with data. Ingeneral, modulation symbols are sent in the frequency domain with OFDMand in the time domain with SC-FDM. The spacing between adjacentsubcarriers may be fixed, and the total number of subcarriers (K) may bedependent on the system bandwidth. For example, K may be equal to 72,180, 300, 600, 900, and 1200 for a corresponding system bandwidth of1.4, 3, 5, 10, 15, or 20 megahertz (MHz), respectively. The systembandwidth may also be partitioned into sub-bands. For example, asub-band may cover 1.08 MHz, and there may be 1, 2, 4, 8 or 16 sub-bandsfor a corresponding system bandwidth of 1.4, 3, 5, 10, 15, or 20 MHz,respectively.

Referring now to FIG. 2, there is shown an example of a system that maybe used to enhance the efficiency of use of available bandwidth inwireless communications channels between one or more UEs 102 and one ormore base stations 104, as discussed above with respect to FIG. 1. FIG.2 illustrates one base station 104 and one UE 102 for purposes ofsimplicity of discussion, though it will be recognized that embodimentsof the present disclosure may scale to many more UEs 102 and/or basestations 104. The UE 102 and the base station 104 may communication witheach other at various frequencies. For example, in one embodiment the UE102 and the base station 104 may communicate at sub-6 GHz frequencies,while in another embodiment at above 6 GHz frequencies, to name just twoexamples.

UE 102 broadcasts a sounding reference signal (SRS) 202 that is receivedby base station 104. In an embodiment, the SRS 202 may be anomni-directional transmission, while in another embodiment the SRS 202may be a wide-beam transmission. Upon receipt of the SRS 202, the basestation 104 is able to gather from the SRS 202, either explicitly orimplicitly, channel information for the uplink channel between the UE102 and the base station 104. The base station 104 may then use thatuplink channel information to train its antennas to beamform a downlink204 to the same UE 102.

To derive the most advantage from reciprocity (applying channelinformation obtained from the SRS 202 in the uplink), the base station104 may rapidly re-apply that information (by training) for beamforming(or focusing) a downlink transmission to the UE 102 so as to minimizethe effects of channel decorrelation. To assist in therapid-reapplication of the channel information in the downlink,embodiments of the present disclosure utilize a short subframestructure. Referring now to FIG. 3, an exemplary subframe structure 300is illustrated that operates within a short timeframe so as to minimizethe effects of decorrelation in the channel. In an embodiment, the shorttimeframe may be approximately 500 microseconds, though it may also beshorter or longer than that. The short timeframe allows the base station104 to essentially “freeze” the channel state for the duration of thesubframe, during which the base station 104 may train and form the beamfor the downlink and then provide a downlink burst.

Communications between UE 102 and base station 104 can be divided in thetime domain into subframes (SFs) 300, such as the SF 300 illustrated inFIG. 3. A single subframe is illustrated in FIG. 3 for ease ofillustration; as will be recognized, the structure of the SF 300 isscalable to any number of subframes as necessary or desired. Each SF 300is divided into an uplink (UL) portion 302 and a downlink (DL) portion304, separated by a transition portion U/D. As part of the UL portion302, the UE 102 may send various types of signals to the base station104. These may include, for example, an SRS (used here for transmitbeamforming at the base station and in place of the uplink DMRS), uplinkdata, and optionally requests for information. The transition portionU/D is provided between the UL portion 302 and the DL portion 304.During the DL portion, the base station 104 sends various types ofsignals to the UE 102, including for example a user-equipment referencesignal (UERS) and downlink data (e.g., in a downlink burst).

In some embodiments, the base station 104 may use the SRS in the ULportion 302 derive multiple pieces of information that facilitate thedownlink between the UE 102 and the base station 104. For example, basedon the SRS the base station 104 having multiple antennas is able totrain its antennas to beamform the DL data transmitted back to the UE102 so that, for instance, interference with other wirelesscommunication devices in the range of the base station 104 is reduced.Beamforming relies on information about the channel between the UE 102and the base station 104 that the base station 104 derives from theuplink SRS and then applies to the downlink based on reciprocity. Thebase station 104 can retrain its antennas as the channel changes overtime (e.g., periodically or randomly), for example according tosubsequent SRS received from the UE 102. This may happen, for example,if the UE 102 is moving or if other moving objects enter or leave thearea/interfere with the uplink (or downlink) channel. According toembodiments of the present disclosure, the subframe 300 is provided aspart of a synchronous system, such that the subframe 300 is providedrepeatedly over time so that the base station 104 may retrain the beamsto accommodate for UE 102 motion and channel decorrelation related tothat movement (and/or other influences).

Channel reciprocity may allow the base station 104 to apply informationabout the channel in the UL direction to estimate one or more channelproperties in the DL direction, which can be used to beamform the DLtransmissions. In this manner, the base station 104 can train itsantennas based on the SRS from the UE 102. The SRS may further includeinformation that allows the base station 104 to demodulate data receivedfrom the UE 102 during the UL portion of the SF 300. The base station104 may additionally determine, from the SRS, scheduling informationthat allows the base station 104 to schedule future SFs 300 (e.g.,frequency bands, etc.) for communicating with the UE 102.

In some embodiments, multiplexing may be used to allow the base station104 to communicate with multiple UEs 102 during the DL portion 304 ofone SF 300. Beamforming can be advantageous because it allows the basestation 104 to make use of space division multiplexing alongside othertypes of multiplexing, such as frequency division multiplexing and/orcode division multiplexing. The base station 104 may therefore requestthat multiple UEs 102 send an SRS during one SF 300, allowing the basestation 104 to retrain its antenna beamforming for each UE 102 that itwill communicate with during that SF 300.

Referring now to FIG. 4, there is illustrated an embodiment ofallocation of SF resources of a SF 400 in a multi-user MIMO (MU-MIMO)scenario. In the embodiment of FIG. 4, two UEs 102 are represented bySRS 1, SRS 2, for simplicity of discussion. It will be recognized thatmore UEs 102 may be included in various embodiments. Each UE 102 in theMU-MIMO system may transmit their SRS at the same time and on the samefrequency allocation (i.e., using non-orthogonal physical resources)without collision by using, for example, permutation or scrambling tomake each SRS unique. In this case, the base station 104 may request anSRS from multiple UEs 102 during the same SF 400 by sending a requestduring a DL portion 402 at the beginning of the SF 400. This request mayinclude information instructing the UEs 102 how to scramble or permutetheir particular SRS (e.g., SRS 1 for a first UE 102 and SRS 2 for asecond UE 102) to avoid interference. Alternatively, the UEs 102 maynotice interference from other UEs 102 and decide to use a permutation,scrambling or the like to transmit an SRS using non-orthogonal physicalresources. The UEs 102 may notify the base station 104 during the ULportion of SF 400 which permutation, scrambling or other method theywill use to create a unique SRS.

Referring now to FIG. 5 which illustrates an alternative embodiment,either the UEs 102 or the base station 104 may determine a minimumprocessing gain (PG) needed to compensate for a poor channel, forexample when a UE 102 is distant from a base station 104. The UE 102 maydetermine a minimum PG by monitoring how long it takes to successfullyreceive a SYNC signal from the base station 104. The base station 104may determine a minimum PG by monitoring how long it takes to set up arandom access channel (RACH) with the UE 102.

In order to achieve the minimum PG, the length of the SRS may need to bescaled to exceed the portion of the SF 500 allocated to the UL portion502. The base station 104 may request an elongated SRS (illustrated inFIG. 5 as SRS 2) from a UE 102 during a DL portion 504 at the beginningof the SF 500, or alternatively the UE 102 may notify the base station104 during the UL portion 502 of SF 500 that it needs to send anelongated SRS. However, as illustrated in SF 500, the UE 102 may stillbe able to transmit its elongated SRS using non-orthogonal physicalresources because there is no danger of its low power signal affectingother UEs 102 in the environment. Therefore, the base station 104 neednot instruct other UEs 102 in the environment to modify their behavior,nor do the other UEs 102 in the environment need to proactively modifytheir behavior.

In another embodiment, a single UE 102 with multiple antennas, such asin a single user MIMO (SU-MIMO) system, may send an SRS from each of itsantennas simultaneously and on the same frequency (i.e., usingnon-orthogonal physical resources) without collision by usingpermutation, scrambling, or a different precoder across antennas to makeeach SRS at each antenna unique from the others at the other antennas.The SF 400 of FIG. 4 (originally described with respect to singleantennas on multiple UEs 102) illustrates this embodiment, as multipleantennas on a single UE 102 function similarly to single antennas onmultiple UEs 102. In this case (referring now to FIG. 4 for thisalternative embodiment), the base station 104 may notify the UE 102during a DL portion 402 at the beginning of the SF 400 how to create aunique SRS for each antenna, or alternatively the UE 102 may choose itsown unique SRS for each antenna and notify the base station 104 what tolook for.

Referring now to FIG. 6, there is illustrated another MU-MIMOembodiment. In this embodiment, multiple UEs 102 (represented as users1, 2, 3, and 4) may send respective SRS during the same SF 600.According to the embodiment of FIG. 6, the multiple UEs 102 may useunique sets of time and frequency allocations, i.e. using orthogonalphysical resources, for the SRS from each respective UE 102. This may benecessary when UEs 102 are close to the base station 104, which resultsin very high power signals received at the base station 104, i.e. a veryhigh UL signal-to-noise ratio (SNR). Such powerful signals could causeinterference with each other even when using scrambling or permutations,so the signals may be allocated onto orthogonal physical resources toavoid collision. The allocations may be of contiguous portions of thesystem bandwidth. Alternatively, the allocations may be spaced outacross tones to leave bandwidth in between the portions used by the UEs102. The allocations need not be symmetrical between UEs 102. Forexample, as shown in SF 600, first and second UEs 102 may each beallocated two non-contiguous pieces of frequency spectrum within a firsttime period (represented by SRS 1 and SRS 2, respectively), while thirdand fourth UEs 102 may be each allocated contiguous blocks of frequencyspectrum (represented by SRS 3 and SRS 4, respectively) within a secondtime period. In general, UEs 102 may be allocated one or more contiguousor non-contiguous blocks of spectrum over one or more contiguous ornon-contiguous time periods. The base station 104 may recognize, forexample based on a very short time to establish a RACH with the UE 102,that the power level of signals received from the UEs 102 is very highand that orthogonal resources should be used for the SRS from one ormore of the UEs 102. The base station 104 may accordingly sendinstructions to the UEs 102 during a DL portion of the SF 600 allocatingphysical resources for the SRS of each UE 102. Alternatively, a given UE102 may recognize that it has a very high UL SNR, for example based on avery short time to receive a SYNC signal from the base station 104, andmay notify the base station 104 that the UE 102 needs its own allocationof physical resources for its SNR. Alternatively, the UE 102 may suggesta potential allocation to the base station 104.

In another embodiment, a single UE 102 with multiple antennas, such asin a SU-MIMO system, may send an SRS from each of its antennas duringthe same SF 600, but using unique sets of time and frequencyallocations, i.e. using orthogonal physical resources. For example, asshown in SF 600, first and second antennas of UE 102 may each beallocated two non-contiguous pieces of frequency spectrum within a firsttime period (represented by SRS 1 and SRS 2, respectively), while thirdand fourth antennas of UE 102 may be allocated a single contiguous blockof frequency spectrum within a second time period (represented by SRS 3and SRS 4, respectively). In this case, the base station 104 may notifythe UE 102 during a DL portion 602 at the beginning of the SF 600 ofresource allocations for each antenna, or alternatively the UE 102 maychoose its own resource allocations for each antenna and notify the basestation 104 what to look for.

Referring now to FIG. 7, there is shown an embodiment where a UE 102 hasa narrowband power amplifier (PA). In order to take advantage of channelreciprocity, which allows the base station 104 to beamform the DLchannel based on the SRS of the UE 102, the SRS may need to cover theentire system bandwidth. If the UE 102 has a narrowband PA, it can onlycover a sub-band of the system bandwidth with any given transmission. Asillustrated in frame structure 700, the UE 102 may transmit multipleconsecutive narrowband SRS at staggered frequencies that, together,cover the entire system bandwidth. The base station 104 may collect andcombine the multiple consecutive narrowband SRS to obtain completeinformation about the system bandwidth of the downlink channel.

Referring now to FIG. 8, according to embodiments of the presentdisclosure information about some portion of the system bandwidth lessthan the whole may be sufficient for channel reciprocity to hold. The UE102 may accordingly transmit only as many staggered narrowband SRS asnecessary to reach the threshold for channel reciprocity, as illustratedby SF structure 800.

FIG. 9 is a flowchart illustrating an exemplary method 900 for using anuplink sounding reference signal for channel estimation in accordancewith various aspects of the present disclosure. The method 900 may beimplemented in the base station 104. The method 900 will be describedwith respect to a single base station 104 for simplicity of discussion,though it will be recognized that the aspects described herein may beapplicable to any number of base stations 104. It is understood thatadditional method blocks can be provided before, during, and after theblocks of method 900, and that some of the blocks described can bereplaced or eliminated for other embodiments of the method 900.

At block 902, a base station 104 receives an SRS from a UE 102 in anuplink communication, as described according to the various embodimentsabove. For example, the base station 104 may receive the SRS as part ofan uplink portion of a subframe as illustrated in FIG. 3. According tothe various embodiments of the present disclosure, the base station 102may receive a single SRS from a single-antenna UE 102, multiple SRScorresponding to multiple antennas of a single UE 102, multiple SRScorresponding to single antennas of multiple UEs 102, and/or multipleSRS corresponding to multiple antennas of multiple UEs 102. Further, theSRS may be provided to the base station 104 according to non-orthogonalor orthogonal SRS, depending upon embodiment.

At block 904, the base station 104 extracts information about the uplinkfrom the SRS received at block 902. This may include information usefulin demodulating uplink data including in the uplink portion of thesubframe, scheduling information, and channel information about theuplink channel.

At block 906, the base station 104 schedules the downlink communication(e.g., the downlink burst that is part of the downlink portion of asubframe), based on information extracted from the SRS at block 904.

At block 908, the base station 104 trains the beamforming for the one ormore antennas of the base station 104 based on channel informationextracted from the SRS received from the UE 102. Based on the SRS, thebeamforming may be invariant to the number of antennas within thesystem, rendering embodiments of the present disclosureforward-compatible with future technologies that include more antennas(e.g., 16, 32, etc.) in MIMO arrays for example.

At block 910, as part of the same subframe, the base station 104transmits a downlink burst including one or more reference signals (suchas a UERS) as well as downlink data. With the beam forms of the antennasof the base station 104 trained based on the channel information derivedfrom the uplink SRS, applied to the downlink by taking advantage ofreciprocity during a short timeframe encapsulated by the subframe, thebase station 104 is able to more improve its utilization of higherfrequencies while still providing a substantially equivalent range thatis possible with lower frequencies/evolution technologies (2G, 3G, 4Gfor example).

It is understood that method 900 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 902-910 upon reading the codefrom the computer readable medium. In some embodiments, the UE 102 andbase station 104 of the present disclosure may include such a processorand such a computer readable medium with program code stored in it.

Turning now to FIG. 10, a flowchart is illustrated of an exemplarymethod 1000 for using a non-orthogonal uplink sounding reference signalfor channel estimation in accordance with various aspects of the presentdisclosure. The method 1000 may be implemented in a UE 102. The method1000 described is applicable to both single UEs 102 having multipleantennas and multiple UEs 102 that each have single antennas. It isunderstood that additional method blocks can be provided before, during,and after the blocks of method 1000, and that some of the blocksdescribed can be replaced or eliminated for other embodiments of themethod 1000.

At block 1002, the UE 102 monitors the interference level. For a singleUE 102 having multiple antennas, this involves monitoring theinterference level at each antenna of the UE 102. For multiple UEs 102that each have a single antenna, this involves each UE 102 monitoringthe interference level of its antenna.

At block 1004, the UE 102 (where multiple antennas) or UEs 102 (whereeach has a single antenna) determines whether a permutation(interleaving) or a scrambling code will better overcome theinterference monitored at block 1002. For example, this may involve theUE 102 determining to use non-orthogonal coding where the UE 102 ispower limited (e.g., the uplink SNR is low) or to enable MU-MIMO on thedownlink (e.g., where the multiple UEs 102 each have multiple antennas).

At block 1006, in response to the determination at block 1004, the UE102 configures the SRS for each of its antennas (or, for single-antennaUEs 102, each UE 102 for its respective antenna) with the uniquepermutation or scrambling code, as determined at block 1004.

With the SRS (of each antenna for a MIMO UE 102 or each antenna for eachUE 102, depending upon embodiment) scrambled, at block 1008 the UE 102(each SRS for each antenna or each UE 102) transmits the scrambled SRSto the base station 104 via the uplink channel. In an embodiment, thetransmission may be done using the full channel bandwidth and the fulluplink subframe portion (as discussed with respect to FIG. 3 above).

After the base station 104 receives the SRS in the uplink portion of thesubframe from the multiple antennas of the UE 102 (or each antenna ofeach UE 102, depending upon the embodiment), the base station 104derives channel state information from the SRS for the uplink channeland, based on reciprocity, applies the derived channel state informationto the downlink channel. This includes training the beamform for theantennas of the base station 104 toward the UE 102.

As a result, at block 1010 the UE 102 receives a beamformed downlinkburst from the base station 104 (at the multiple antennas of a single UE102 or at each antenna of each UE 102 of many) as part of the downlinkportion of the same subframe.

It is understood that method 1000 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1002-1010 upon reading thecode from the computer readable medium. In some embodiments, the UE 102and base station 104 of the present disclosure may include such aprocessor and such a computer readable medium with program code storedin it.

FIG. 11 illustrates a flowchart of an exemplary method 1100 for using anorthogonal uplink sounding reference signal for channel estimation inaccordance with various aspects of the present disclosure. The method1100 may be implemented in a UE 102. The method 1100 described isapplicable to both single UEs 102 having multiple antennas and multipleUEs 102 that each have single antennas. It is understood that additionalmethod blocks can be provided before, during, and after the blocks ofmethod 1100, and that some of the blocks described can be replaced oreliminated for other embodiments of the method 1100.

At block 1102, the UE 102 monitors the interference level. For a singleUE 102 having multiple antennas, this involves monitoring theinterference level at each antenna of the UE 102. For multiple UEs 102that each have a single antenna, this involves each UE 102 monitoringthe interference level of its antenna, as described above with respectto FIG. 10.

At block 1104, the UE 102 (where multiple antennas) or UEs 102 (whereeach has a single antenna) determines whether the uplink SNR issufficiently high to allow the SRS to be orthogonal—where each SRS ateach antenna (either at a single UE 102 or multiple UEs 102) isallocated a different time/frequency combination physical resource.

At block 1106, in response to the determination at block 1104, the UE102 configures the SRS for each of its antennas (or, for single-antennaUEs 102, each UE 102 for its respective antenna) with particularfrequency/time combinations. For example, the frequency which each SRSis allocated may be contiguous to other frequencies assigned to otherSRS or may be staggered across tones.

With the SRS of each antenna for a MIMO UE 102 (or each antenna for eachUE 102, depending upon embodiment) assigned a different frequency/timephysical resource, at block 1108 the UE 102 (each SRS for each antennaor each UE 102) transmits the SRS to the base station 104 via the uplinkchannel using the unique frequency/time physical resources.

After the base station 104 receives the SRS in the uplink portion of thesubframe from the multiple antennas of the UE 102 (or each antenna ofeach UE 102, depending upon the embodiment), the base station 104derives channel state information from the SRS for the uplink channeland, based on reciprocity, applies the derived channel state informationto the downlink channel. This includes training the beamform for theantennas of the base station 104 toward the UE 102.

As a result, at block 1110 the UE 102 receives a beamformed downlinkburst from the base station 104 (at the multiple antennas of a single UE102 or at each antenna of each UE 102 of many) as part of the downlinkportion of the same subframe.

It is understood that method 1100 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1102-1110 upon reading thecode from the computer readable medium. In some embodiments, the UE 102and base station 104 of the present disclosure may include such aprocessor and such a computer readable medium with program code storedin it.

Turning now to FIG. 12, a flowchart is illustrated of an exemplarymethod 1200 for using an uplink sounding reference signal for channelestimation in accordance with various aspects of the present disclosure.The method 1200 may be implemented in a UE 102 that has a narrowbandpower amplifier. It is understood that additional method blocks can beprovided before, during, and after the blocks of method 1200, and thatsome of the blocks described can be replaced or eliminated for otherembodiments of the method 1200.

At block 1202, the UE 102 determines whether the power amplifier isnarrowband. As described above, a narrowband power amplifier may onlycover a sub-band of the system bandwidth with any given transmission.

At block 1204, in response to determining that the power amplifier ofthe UE 102 is narrowband, the UE 102 generates and transmits a series ofconsecutive SRS that are staggered across frequencies across a largeportion of or the entire system bandwidth, for example as illustrated inFIG. 7, as part of the uplink portion of a subframe according to theembodiments discussed above.

In response, the base station 104 receives the consecutive SRS (in time,staggered across frequencies) and combines to obtain a substantiallyfull view of the uplink channel information. The base station 104, usingreciprocity, then in turn applies the channel information to thedownlink channel and beamforms the antennas accordingly.

At block 1206, the UE 102 receives the beamformed downlink burst as partof the same subframe from the base station 104.

It is understood that method 1200 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1202-1206 upon reading thecode from the computer readable medium. In some embodiments, the UE 102and base station 104 of the present disclosure may include such aprocessor and such a computer readable medium with program code storedin it.

FIG. 13 is a block diagram of an exemplary wireless communication device1300 according to embodiments of the present disclosure. The wirelesscommunication device 1300 may be a UE 102 as discussed above. As shown,the UE 102 may include a processor 1302, a memory 1304, an SRSconfiguration module 1308, a transceiver 1310 (including a modem 1312and RF unit 1314), and an antenna 1316. These elements may be in director indirect communication with each other, for example via one or morebuses.

The processor 1302 may include a central processing unit (CPU), adigital signal processor (DSP), an application-specific integratedcircuit (ASIC), a controller, a field programmable gate array (FPGA)device, another hardware device, a firmware device, or any combinationthereof configured to perform the operations described herein withreference to UEs 102 introduced above with respect to FIG. 1 anddiscussed in more detail above. In particular, the processor 1302 may beutilized in combination with the other components of the UE 102,including correlation information module 1308, to perform the variousfunctions associated with orthogonal or scrambled SRS as described ingreater detail above. The processor 1302 may also be implemented as acombination of computing devices, e.g., a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The memory 1304 may include a cache memory (e.g., a cache memory of theprocessor 1302), random access memory (RAM), magnetoresistive RAM(MRAM), read-only memory (ROM), programmable read-only memory (PROM),erasable programmable read only memory (EPROM), electrically erasableprogrammable read only memory (EEPROM), flash memory, solid state memorydevice, hard disk drives, other forms of volatile and non-volatilememory, or a combination of different types of memory. In an embodiment,the memory 1304 includes a non-transitory computer-readable medium. Thememory 1304 may store instructions 1306. The instructions 1306 mayinclude instructions that, when executed by the processor 1302, causethe processor 1302 to perform the operations described herein withreference to the UEs 102 in connection with embodiments of the presentdisclosure. Instructions 1306 may also be referred to as code. The terms“instructions” and “code” should be interpreted broadly to include anytype of computer-readable statement(s). For example, the terms“instructions” and “code” may refer to one or more programs, routines,sub-routines, functions, procedures, etc. “Instructions” and “code” mayinclude a single computer-readable statement or many computer-readablestatements.

The SRS configuration module 1308 may be used for various aspects of thepresent disclosure. For example, the SRS configuration module 1308 maybe used to measure interference at the antenna or antennas of the UE102. In one embodiment, the SRS configuration module 1308 may thendetermine whether permutation or scrambling will overcome the measuredinterference, and configure an SRS for each antenna with uniquepermutation or scrambling. In another embodiment, the SRS configurationmodule 1308 may determine whether use of orthogonal time and frequencyresources (i.e. physical channel resources) for SRS transmission isnecessary, and it may configure each antenna of the UE 102 to useorthogonal time and frequency resources for SRS transmission.

As shown, the transceiver 1310 may include the modem subsystem 1312 andthe radio frequency (RF) unit 1314. The transceiver 1310 can beconfigured to communicate bi-directionally with other devices, such asbase stations 104. The modem subsystem 1312 may be configured tomodulate and/or encode the data from the correlation information 1308and other aspects of the UE 102, such as processor 1302 and/or memory1304, according to a modulation and coding scheme (MCS), e.g., alow-density parity check (LDPC) coding scheme, a turbo coding scheme, aconvolutional coding scheme, etc. The RF unit 1314 may be configured toprocess (e.g., perform analog to digital conversion or digital to analogconversion, etc.) modulated/encoded data from the modem subsystem 1312(on outbound transmissions) or of transmissions originating from anothersource such as a UE 102 or a base station 104. Although shown asintegrated together in transceiver 1310, the modem subsystem 1312 andthe RF unit 1314 may be separate devices that are coupled together atthe UE 102 to enable the UE 102 to communicate with other devices.

The RF unit 1314 may provide the modulated and/or processed data, e.g.data packets (or, more generally, data messages that may contain one ormore data packets and other information), to the antenna 1316 fortransmission to one or more other devices. This may include, forexample, transmission of . . . according to embodiments of the presentdisclosure. The antenna 1316 may further receive data messagestransmitted from other devices and provide the received data messagesfor processing and/or demodulation at the transceiver 1310. AlthoughFIG. 13 illustrates antenna 1316 as a single antenna, antenna 1316 mayinclude multiple antennas of similar or different designs in order tosustain multiple transmission links.

FIG. 14 illustrates a block diagram of an exemplary base station 104according to the present disclosure. The base station 104 may include aprocessor 1402, a memory 1404, a beamforming module 1408, a transceiver1410 (including a modem 1412 and RF unit 1414), and an antenna 1416.These elements may be in direct or indirect communication with eachother, for example via one or more buses.

The processor 1402 may have various features as a specific-typeprocessor. For example, these may include a CPU, a DSP, an ASIC, acontroller, a FPGA device, another hardware device, a firmware device,or any combination thereof configured to perform the operationsdescribed herein with reference to the base stations 104 introduced inFIG. 1 above. The processor 1402 may also be implemented as acombination of computing devices, e.g., a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The memory 1404 may include a cache memory (e.g., a cache memory of theprocessor 1402), RAM, MRAM, ROM, PROM, EPROM, EEPROM, flash memory, asolid state memory device, one or more hard disk drives, memristor-basedarrays, other forms of volatile and non-volatile memory, or acombination of different types of memory. In some embodiments, thememory 1404 may include a non-transitory computer-readable medium. Thememory 1404 may store instructions 1406. The instructions 1406 mayinclude instructions that, when executed by the processor 1402, causethe processor 1402 to perform operations described herein with referenceto a base station 104 in connection with embodiments of the presentdisclosure. Instructions 1406 may also be referred to as code, which maybe interpreted broadly to include any type of computer-readablestatement(s).

The beamforming module 1408 may be used for various aspects of thepresent disclosure. For example, the beamforming module 1408 may beinvolved in extracting information from a SRS received from a UE 102 andusing the extracted information to train beamforming for the one or moreantennas 1416 for a downlink with the UE 102.

As shown, the transceiver 1410 may include the modem subsystem 1412 andthe radio frequency (RF) unit 1414. The transceiver 1410 can beconfigured to communicate bi-directionally with other devices, such asUE 102 and/or another core network element. The modem subsystem 1412 maybe configured to modulate and/or encode data according to a MCS, e.g., aLDPC coding scheme, a turbo coding scheme, a convolutional codingscheme, etc. The RF unit 1414 may be configured to process (e.g.,perform analog to digital conversion or digital to analog conversion,etc.) modulated/encoded data from the modem subsystem 1412 (on outboundtransmissions) or of transmissions originating from another source suchas a UE 102. Although shown as integrated together in transceiver 1410,the modem subsystem 1412 and the RF unit 1414 may be separate devicesthat are coupled together at the base station 104 to enable the basestation 104 to communicate with other devices.

The RF unit 1414 may provide the modulated and/or processed data, e.g.data packets (or, more generally, data messages that may contain one ormore data packets and other information), to the antenna 1416 fortransmission to one or more other devices. This may include, forexample, use of beamforming to transmit information to a UE 102according to embodiments of the present disclosure. The antenna 1416 mayfurther receive data messages transmitted from other devices and providethe received data messages for processing and/or demodulation at thetransceiver 1410. Although FIG. 14 illustrates antenna 1416 as a singleantenna, antenna 1416 may include multiple antennas of similar ordifferent designs in order to sustain multiple transmission links.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals, bits, symbols, and chips that may bereferenced throughout the above description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general-purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a DSP and a microprocessor, multiple microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of [at least one of A, B, or C]means A or B or C or AB or AC or BC or ABC (i.e., A and B and C).

As those of some skill in this art will by now appreciate and dependingon the particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the spirit and scope thereof. In lightof this, the scope of the present disclosure should not be limited tothat of the particular embodiments illustrated and described herein, asthey are merely by way of some examples thereof, but rather, should befully commensurate with that of the claims appended hereafter and theirfunctional equivalents.

What is claimed is:
 1. A method for communicating with a base station,comprising: determining, by a user equipment (UE), each one of multiplenarrowband sounding reference signals (SRSs) based on a bandwidthcapability of the UE being narrower than a system bandwidth of achannel; transmitting, from the UE, each one of the multiple narrowbandSRSs at a different frequency sub-band during one subframe; andreceiving, by the UE from the base station, a beamformed downlinkcommunication based on information obtained from each one of themultiple narrow band SRSs corresponding to each of the differentfrequency sub-bands, wherein the beamformed downlink communication basedon information obtained from each one of the multiple narrowband SRSscorresponding to each of the different frequency sub-bands comprises abeamformed downlink communication having a bandwidth greater than thewidth of each of the frequency sub-bands, wherein the differentfrequency sub-bands cover a portion of an entire bandwidth of the systembandwidth of the channel, wherein a number of different frequencysub-bands is selected based on a threshold for channel reciprocity. 2.The method of claim 1, wherein the different frequency sub-bands coveran entire bandwidth of the system bandwidth of the channel.
 3. Themethod of claim 1, wherein transmitting each one of the multiplenarrowband SRSs at a different frequency sub-band during one subframecomprises transmitting each one of the multiple narrowband SRSs during adifferent time resource of the one subframe.
 4. A user equipment (UE),comprising: a processor configured to determine each one of multiplenarrowband sounding reference signals (SRSs) based on a bandwidthcapability of the UE being narrower than a system bandwidth of achannel; and a transceiver configured to: transmit each one of themultiple narrowband SRSs at a different frequency sub-band during onesubframe; and receive from a base station a beamformed downlinkcommunication based on information obtained from each one of themultiple narrowband SRSs corresponding to each of the differentfrequency sub-bands, wherein the beamformed downlink communication basedon information obtained from each one of the multiple narrowband SRSscorresponding to each of the different frequency sub-bands comprises abeamformed downlink communication having a bandwidth greater than thewidth of each of the frequency sub-bands, wherein the differentfrequency sub-bands cover a portion of an entire bandwidth of the systembandwidth of the channel, wherein a number of different frequencysub-bands is selected based on a threshold for channel reciprocity. 5.The user equipment of claim 4, wherein the different frequency sub-bandscover an entire bandwidth of the system bandwidth of the channel.
 6. Theuser equipment of claim 5, wherein transmitting each one of the multiplenarrowband SRSs at a different frequency sub-band during one subframecomprises transmitting each one of the multiple narrowband SRSs during adifferent time resource of the one subframe.